Many well-defined mechanical devices are available for use in vehicle propulsion. Some, however, do not lend themselves to use in-present day drive train design, because vehicle speed is controlled by engine speed, vehicle speed being directly related to engine speed by the mechanical transmission which adjusts the gear ratio between the driving wheels and the engine to the best value for the torque/speed requirement. Decoupling vehicle speed (wheel RPM) from engine speed permits the use of constant speed engines or even variable speed engines where engine speed is determined by factors other than vehicle speed such as power demand.
Historically, it has been demonstrated that vehicles powered by "on-off" engines can achieve superb fuel mileage. Such engines comprise one or more cylinders which can separately operate in one of two selectable states: either at a predetermined "on" speed (typically corresponding to the most efficient operating speed of the engine and termed its "on" mode) or at an "off," or zero, speed (termed its "off" mode). Shell Oil Company has sponsored mileage contests for years. In 1977 (See Popular Science, December 1977, page 100) an entry by Cranfield Institute of Technology achieved 914 miles on one gallon of gasoline by careful management of the vehicle speed i.e. speeding up, coasting (engine off), speeding up, coasting, etc. throughout the race. Even though this demonstrates good energy management, this technique of driving would be totally unacceptable to the driving public.
Considerable energy is lost in heat dissipation every time a vehicle is brought to a stop. Regenerative braking has been in use for years in railway cars where the electric drive system permits the reclamation of some of the kinetic energy by pumping it back into the supply system. This method is impractical for single vehicles isolated from a large energy system; however, other well known short term storage devices such as the flywheel are available for use in energy storage. To take advantage of short term storage i.e. the flywheel, and prime power sources such as the "on-off" engine, a means of controlling vehicle speed other than by engine speed must be used. A transmission capable of coupling the energy from the power source to the drive wheels so that the torque and wheel speed requirement is met is needed. Many forms of variable speed transmissions are available that might fill this need but the well known Ward-Leonard system which uses a D.C. generator and a D.C. motor provides excellent speed control of the motor while the generator can be either a fixed speed or variable speed and not related to the motor speed.
A judicious combination of the best qualities of available technology can provide a system that is capable of greatly improved efficiency.